DDOC-684: Align retention policy docs with SDKs

[bszwarc]

Description

Add improvements to the retention policy docs so that they match SDK documentation and samples. Preview: https://staging.developer.box.com/reference/resources/retention-policy/ https://staging.developer.box.com/reference/resources/retention-policy--base/ https://staging.developer.box.com/reference/resources/retention-policy-assignment--base/ https://staging.developer.box.com/reference/get-retention-policies-id/ https://staging.developer.box.com/reference/get-retention-policy-assignments-id/ https://staging.developer.box.com/reference/resources/retention-policies--mini/

Fixes DDOC-684